In helicopters and other vertical take-off and landing vehicles, the landing gear may comprise wheel assemblies to facilitate ground taxiing of the aircraft. One common wheel arrangement utilized for helicopter landing gear is a tripod arrangement having a main landing gear forward or aft the the aircraft center of gravity and a corresponding secondary landing gear (tail landing gear or nose landing gear) at the tail or nose of the aircraft, respectively. The main landing gear comprises two wheel assemblies on either side of the aircraft, each having an independently operable braking system, e.g., disk brakes. The secondary landing gear is mounted in a caster type arrangement such that the center axis of the wheel is offset from the center axis of the landing gear. This caster arrangement allows the wheel to rotate for easily moving the aircraft in any direction.
The secondary landing gear is also provided with a locking mechanism (shear pin) which may be manually operable or remotely operable by means of an electronic or mechanical actuator for repositioning the shear pin between a locked and unlocked position. The locking mechanism is typically maintained in the locked position with the landing gear in a fixed position orientated for aircraft forward movement. This facilitates certain ground taxiing, takeoff and landing maneuvers because side movement of the aircraft is prevented. Additionally, if the aircraft is moving rapidly, locking the wheel in the forward position helps prevent the aircraft from going into a spin if the main landing gear brakes are applied unevenly. If the secondary landing gear is retractable, the locking mechanism also holds the landing gear in the proper position for retraction. As is known in the art, retractable landing gear is typically provided with a mechanism to properly orient the rotating portion of the landing gear prior to retraction if the locking mechanism is not engaged.
Helicopter landing gear is typically provided with energy absorbing devices, such as shock struts, to absorb the vertical impact which the landing gear normally encounters in making a landing. Such energy absorbing devices are generally adequate to absorb the vertical loading on the landing gear when landing on surfaces specifically designed for aircraft landings, e.g., an aircraft landing field, helicopter pad, etc. However, because helicopters are capable of landing in almost any cleared area, caster type landing gear may encounter conditions of excessive side loading, which could occur for example when the landing gear encounters some obstruction on the landing surface, e.g., a rut, large rock, tree limb, etc. Since the shock strut is not designed to absorb the large side loading, the landing gear may be stressed beyond its design limits so that it and possibly other components of the aircraft would be damaged.
To prevent damage to the secondary landing gear caused by excessive side loading, the above-mentioned shear pins are utilized in tail and nose landing gear mechanisms. The shear pins are designed to fail under conditions of excessive side loading to prevent overstressing of certain components of the aircraft and diminish the impact which is ultimately transmitted to the aircraft. Upon failure of the shear pin, the lower portion of the landing gear is allowed to turn freely about the axis of the shock strut, thereby permitting the wheel to become oriented such that the landing gear is no longer subject to excessive side loading. However, the ability of the shear pin to fail during landing results in the need for frequent replacement of the pins, thereby making the aircraft unavailable during the repair period.